Method for injection molding of plastic materials using gas holding pressure in mold

ABSTRACT

A system and process for gas-assisted injection molding of plastic materials using gas holding pressure in the mold cavity. An infinitely controlled gas pressure valve is utilized to provide initial pressure in a sealed mold and the gas is vented to a prespecified pressure when plastic is injected into the mold cavity. Additional gas is injected into the plastic material from a gas pin assembly in order to force the plastic to completely fill the mold cavity and to provide a hollow cavity in the completed part. Once the injection molding process has been completed and the part has cooled and solidified, the mold is opened, and the completed plastic molded component or part is removed or ejected.

TECHNICAL FIELD

[0001] The present invention relates to plastic injection moldingsystems and more particularly to gas-assisted plastic injection moldingsystems and processes.

BACKGROUND OF THE INVENTION

[0002] There are numerous known systems for plastic injection molding.In conventional plastic injection molding systems, plastic pellets aremelted in an injection molding machine and advanced by a screw ram intoa mold cavity. The mold cavity is formed between two mold halves (a coremember and a cavity member), typically through one or more spruebushings, a manifold and/or a hot runner system. The two halves of themold are clamped, typically under high pressure, and the plastic isinjected into the mold cavity, again under significant pressure in mostinstances. The molten plastic material in the cavity is allowed to cooland harden in the cavity, typically by a cooling system which circulatesa cooling fluid through one or more of the mold members. When the parthas sufficiently hardened, the mold is opened and the part is removed,typically by one or more ejector pins.

[0003] Some of the known systems utilize a gas in the injection moldingprocess and are commonly known as “gas-assisted injection molding”systems. In these systems, the gas is injected into the molten plasticmaterial through the plastic injection nozzle itself, or through one ormore pin mechanisms strategically positioned in the mold, spruebushings, manifold or hot runner systems. It is also possible to injectthe gas directly into the molten plastic in the barrel of the injectionmolding machine. The gas, which typically is an inert gas such asnitrogen, is injected under pressure and forms one or more hollowcavities or channels in the molded part. The benefits of gas-assistedinjection molding processes are well-known, and include the cost savingsthrough the use of less plastic material, producing parts which arelighter in weight, and producing parts which have better surfacedefinitions and finishes.

[0004] Another plastic injection molding system which utilizes gasinjects the gas into the mold cavity along one or more exterior surfacesof the molded part. The pressurized gas forces the plastic against theopposite surface or surfaces of the mold cavity and forms a part withsuperior surface characteristics on the appearance surfaces.

[0005] One particular gas-assisted injection molding system utilizes aconnecting spill-over cavity coupled to the mold cavity. Such system isshown, for example, in U.S. Pat. No. 5,098,637. In this system, aportion of the plastic from the mold cavity is displaced into thespill-over cavity when the charge of pressurized gas is introduced. Thisprocess has particular use for door and grip handles.

[0006] Although many of these gas-assisted injection molding systemsoperate satisfactorily and have produced commercially acceptable plasticinjection molded parts and components, there is a need for improvedsystems and processes, and particularly those which do not utilizespill-over cavities.

SUMMARY OF THE INVENTION

[0007] It is an object of the present invention to provide an improved“gas-assisted injection molding” system and process. It is anotherobject of the present invention to provide an improved gas-assistedinjection molding process and system for making plastic parts andcomponents in which a spill-over cavity is not utilized or required.

[0008] It is another object of the present invention to provide agas-assisted injection molding system and process in which the amount ofplastic material injected into the mold and the pressures of theinjected gas are controlled. It is a still further object of the presentinvention to provide a gas-assisted injection molding system and processin which the gas pressures in the mold are controlled in certain mannersin order to provide a satisfactory product in an improved and beneficialmanner.

[0009] In accordance with the present invention, a sealed mold isutilized, the plastic is injected from a standard injection moldingmachine, and the gas is injected into the mold either through themachine nozzle or through one or more gas pin assemblies. The mold isinitially pressurized by the same or a separate gas source to aprespecified pressure. An infinitely pressure-controlled valve isutilized to control the pressure of gas in the mold. The plasticmaterial is injected into the mold until a preselected pressure level isreached at which time the gas pressure valve holds the gas pressureconstant. Alternatively, the gas pressure valve can allow the gaspressure to decay in a controlled manner

[0010] At a preset distance of travel of the screw ram in the injectionmolding machine, a gas is injected through the gas pin assembly orassemblies. Alternatively, as the initial gas pressure is increased bythe introduction of the plastic material and reaches a pre-selectedlevel, the gas can be injected through the gas pin assembly orassemblies. The remaining portion of the plastic needed to make themolded part is also injected into the cavity.

[0011] Once the plastic has been injected, the electrically controlledgas pressure valve will reduce the pressure in order to allow theplastic to fill the extremities of the cavity. Also, gas injectionpressure through the gas pin assembly or assemblies will assist indeploying the plastic and regulating the final gas pressure.

[0012] Once the plastic product has cooled, gas is vented from the mold,the mold is opened, and the part is removed from the mold. Preferably,one or more ejector pins are utilized to assist in removing the partfrom the mold.

[0013] Other embodiments, benefits, and features of the presentinvention will become apparent from the following description of theinvention when viewed in accordance with the accompanying drawings andappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a schematic illustration of a preferred embodiment ofthe present invention;

[0015] FIGS. 2-5 depict various steps in the process in accordance withthe present invention;

[0016]FIG. 6 illustrates a combined gas entry and ejector pin assemblywhich can be utilized with the present invention;

[0017]FIG. 7 is a cross-sectional view taken along line 7-7 in FIG. 6and in the direction of the arrows; and

[0018]FIG. 8 is a flow chart depicting the elements of the inventiveprocess.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0019] A preferred embodiment of the present invention is shown in FIGS.1-5 and indicated generally by the reference numeral 10. These figuresshow both a unique and inventive system for gas-assisted injectionmolding as well as illustrating a unique and inventive process forgas-assisted injection molding.

[0020] The embodiment 10 includes a mold 12 which includes a first uppermold member 14 and a second or lower mold member 16. A cavity 18 isformed in the mold 12 and is formed or machined in the precise shape ofthe final plastic component or part to be manufactured. An injectionmolding machine 20 is positioned adjacent the mold 12 and includes anozzle member 22 which is pressed in contact with the mold 12. Inaccordance with conventional injection molding processes, molten plasticmelted in the injection molding machine is injected through the nozzle22 and into the mold cavity 18 in the mold 12.

[0021] Also in accordance with the preferred embodiment 10, pressurizedgas, such as nitrogen, is injected into the mold cavity 18 through oneor more gas pin assemblies 30. The gas is inserted through the gasassembly 30 from a gas source 32.

[0022] Also in the embodiment 10, one or more pneumatic or hydraulicejector pin assemblies 34 are provided. As with conventional injectionmolding processes, the ejector pin assemblies are utilized to force oreject the molded part from the mold cavity once the part has been formedand hardened.

[0023] In accordance with the present invention, the mold 12 is sealed.This is shown schematically by the presence of seals 36 and 38 which arepositioned in grooves between the two mold members 14 and 16. It isunderstood, of course, that the mold cavity can be sealed in anyconventional manner and that the use of O-rings as described is only oneway to accomplish this. Any other conventional mechanisms or means forsealing a mold cavity can be utilized in accordance with the knowledgeof persons of ordinary skill in the art.

[0024] An electrically controlled gas pressure valve 40 is connected toconduit 42 and used to control the pressure of the gas in the moldcavity 18. The gas pressure valve 40 is infinitely pressure controlledand can be of any conventional type, such as a Tescom servovalve.

[0025] A directional valve 46 is also provided and in communication withconduit 42 and the mold cavity 18 through conduit 48. A gas source 33 isalso connected through the valve 46 to the conduit 42 and thus to themold cavity 18. This allows pressurization of the mold cavity as desiredand in accordance with the present invention. In this regard, the gassources 32 and 33 can be separate, as shown, or comprise the same gassource as desired. If the same gas source is utilized, then pressureregulators and the like would have to be utilized in order to allow gasto be directed to the gas pin assemblies 30 and through the valve 46 indifferent amounts and at different times as required by the presentinvention.

[0026] It is to be understood that any type of sealed mold member can beutilized in accordance with the present invention. The particular mold12 shown in FIG. 1 is only illustrative of the various molds that can beutilized in accordance with the present invention. Similarly, any typeof gas pin assembly or assemblies 30, ejector pin assembly or assemblies34, vent valve mechanism 46, injection molding machine 20, and machinenozzle 22 can be utilized, so long as the objectives and purposes of thepresent invention are met. Different components of this type are withinthe skill of persons of ordinary skill in the art. Also, anyconventional type of gas source or sources can be utilized.

[0027] Although the gas to be introduced into the mold cavity 18 isshown to be injected through gas assembly or assemblies 30, it isunderstood that the gas for the gas-assisted injection molding processcan also be provided through any other conventional manner, such as inthe machine nozzle 22. This is shown in hidden lines by gas source 32′in FIG. 1. Also, it is to be understood that any number of gas pinassemblies can be utilized with the present invention. The type of gaspin assembly and the number of assemblies utilized depend on the sizeand configuration of the mold cavity 18 and the resulting part orcomponent to be produced.

[0028]FIGS. 2 through 5 illustrate a preferred process in accordancewith the present invention. As shown in FIG. 2, the mold 10 is firstclosed and clamped together in a conventional manner. The seals 36, 38provide a sealed mold so that pressure cannot escape from it. Initially,the pressure in the mold is energized to a pressure of P1. This pressureis a pre-established pressure setting on the gas pressure valve 40. Gasfrom gas source 33 is injected through the valve 46, conduit 48 andconduit 42 into the mold cavity 18. This is shown by arrow 52. The valueof pressure P1 is preestablished by the parameters of the injectionmolding process, including factors such as the amount of material to beutilized, the size of the part and the melt index.

[0029] Thereafter, as shown in FIG. 3, the injection of plastic materialinto the mold cavity 18 is initiated. The plastic material is suppliedby the injection molding machine 20 through the nozzle 22 and into themold cavity. In FIG. 3, the plastic material is generally indicated bythe reference numeral 60.

[0030] As the plastic is introduced into the mold cavity and occupiessome of its volume, the gas pressure in the mold and at the gas pressurevalve 40 will increase to a pressure P2. Also, the plastic injectionpressure will increase.

[0031] When the injected pressure has compressed the pre-charged gaspressure P1 to the maximum pre-selected pressure as set in gas pressurevalve 40, any excess pressure will vent through pressure valve 40. Inthis manner, the pressure is held constant in the mold cavity from thispoint on. For example, if the initial pressure of P1 in the mold cavityis 300 psi, and the desired pressure is 600 psi, the pressure of P2 willreach 600 psi when the mold cavity 18 is fifty percent (50%) filled withthe plastic material 60.

[0032] At a preset distance of travel of the screw ram in the injectionmolding machine 20, or when the initial gas pressure reaches apre-selected level, the gas pin assembly or assemblies 30 will startinjecting gas into the plastic material. This is shown in FIG. 4. Apressure switch is used to initiate the flow of this gas. As the gas isinjected through the pin assembly 30 into the plastic material 60, thegas forces the plastic material into all of the portions of the mold andforms a hollow cavity 62 in the material. The pressure of the gas fromthe gas source 32 and the pin assemblies or assembly 30 is indicated bypressure P3 in FIG. 4. The gas injection pressure P3 will be eitherslightly more or less than the maximum pressure setting of theelectrically controlled gas pressure valve 40. Thereafter, the remainingplastic material 60 is continued to be injected into the mold cavity 18sufficient to make the completed molding component or part. For example,if a weight reduction of 20 percent is desired, than 80 percent of theplastic by volume will be required. The pin assembly or assemblies 30will preferably begin injecting gas into the plastic material when90-98% (and preferably 95-98%) of the plastic material has been injectedinto the mold cavity.

[0033] The distance of travel of the screw ram can be measured ordetermined in any conventional manner, such as linear transducer (LTD)or a limit switch. The back or “contra” gas pressure in the mold cavityprevents the plastic material from “jetting” into the cavity. Instead,the plastic material flows into the mold cavity essentially as a “plug”with a relatively constant cross-section and leading front surface.

[0034] At the stage of the plastic injection process where 90-98% byvolume of the plastic is injected into the mold cavity, the valve 40will begin venting the predetermined maximum gas pressure of P2. Theventing will take place at a controlled rate and pressure. This is shownin FIG. 5. As the gas injection pressure from the gas pin assembly orassemblies 30 continues, the plastic material is forced throughout theextremities and extent of the mold cavity 18. At the same time, the flowof the plastic material will force the gas holding pressure P2 outthrough the valve 40.

[0035] Thereafter, the pressure P3 in the mold will be held constantuntil the plastic material 60 is solidified and cooled sufficient tomake the component or part self-supporting. At that point, the mold 12will be opened and the plastic part or component ejected. The gas pinassembly 34 or plurality of gas pin ejector assemblies 34 will then beactivated in a conventional manner to eject the completed part from themold cavity. These latter steps are all conventional and can beaccomplished in accordance with standard injection molding procedures,mechanisms, and processes.

[0036] When the molded part has cooled and solidified, the gas pressureP3 in the hollow cavity in the molded part is removed or releasedthrough the gas pin assembly or assemblies 30. Thereafter, the mold isopened and the part ejected in a conventional fashion.

[0037] The difference in pressures between P2 and P3, acting againsteach other, eliminates blow out of plastic in the mold and allows theplastic to keep moving in the mold cavity throughout the total injectionphase. This prevents hesitation marks and helps provide a better finishon the final component or part. Hesitation marks are typically caused bythe surging (stopping and starting) of the plastic material in the moldduring molding.

[0038] The gas used to fill the mold, as represented by pressures P1 andP2, can be recycled. This can be done in any conventional manner asknown to persons of ordinary skill in the art. The recycling of gashelps make the process more efficient and less expensive.

[0039] As indicated above, the representation of gas pin assembly 30 isonly schematic and symbolic. Any known gas pin can be utilized inaccordance with the present invention. In the drawings, the gas pin isdepicted protruding into the mold cavity, but this is not required andis just one option.

[0040] Also, as indicated above, the gas pin assembly or assemblies 30can be replaced by introducing gas through the machine nozzle 22, asshown by reference numeral 32′ in FIG. 1. If gas is introduced throughthe nozzle 22, then a sprue brake is utilized in order to vent the gasfrom the molded part prior to opening of the mold and ejection of thepart.

[0041] In accordance with an alternate embodiment, a vacuum can be drawnin the mold cavity 18 prior to pressurization with gas in order toremove any air in the mold. Also, the machine nozzle 22 is preferablysealed or provided with a gas-tight seal against the mold 12 in order toallow the present inventive process to be more efficient.

[0042] Also, the ejector pin assembly or assemblies 34 are sealed toprevent gas leakage. Preferably, O-rings and the like are positionedaround each ejector pin for this purpose. Other sealing systems ormechanisms known to persons of ordinary skill in the art can also beutilized to seal the ejector pin assemblies to prevent leakage of gas.

[0043] A preferred method for use in the present invention is shown inthe flowchart in FIG. 8. The method is referred to generally byreference number 100 and starts with the step of sealing the mold 110.After the mold is sealed, the mold cavity is pressurized with a gas at apreselected pressure. This is step 120. A plastic material is theninjected into the mold cavity until a second pre-selected pressure isreached in the mold cavity. This is shown by step 130.

[0044] Gas is then injected internally into the plastic material. Thisis step 140. Thereafter, the pressurized gas from the mold cavity whichis applying a contra or back pressure force against the plastic materialin the mold cavity is vented, preferably at a controlled rate. This isshown in step 150. The injection of the plastic material and theinjection of the gas into the plastic material are continued at thispoint. Thereafter, injection of the plastic material into the moldcavity is terminated. This is step 160. Subsequently, the gas which isbeing injected into the plastic material is terminated. This is step170. The gas flow is continued into the plastic material until theentire mold cavity is full and the molding process is complete.

[0045] The gas pressure is then held constant in the plastic materialuntil the plastic material is cooled and hardened. This is shown in step180. Thereafter, the internal gas in the hollow plastic article isvented. This is step 190. Finally, the mold is opened and the completedmolded part is removed. This is step 200. Typically, injection pins areutilized in order to assist in the removal of the molded part from themold cavity.

[0046] In another alternate embodiment, a combined gas entry and ejectorpin mechanism can be utilized. Such a mechanism is shown in FIG. 6 andreferred to generally by the reference numeral 70. The mechanism 70includes an upper member 72 and a base member 74. The base 74 isconnected to the lower mold member 16′ by a plurality of fasteners, suchas bolts 76.

[0047] The combined gas entry and ejector pin mechanism 70 is positionedin the lower mold portion 16′ of a mold 12′ which also contains an uppermember 14′. A mold cavity 18′ in which the plastic is to be injected issupplied between the two mold halves 14′ and 16′. Also, in accordancewith the present invention, the mold cavity 18′ is sealed, such as byO-ring members 36′.

[0048] A removable rod or ejector pin member 80 is positioned in themechanism 70 and adapted to be activated by gas from gas source 82 to beinjected or forced into the mold cavity 18′ and thus eject the completedmolded plastic component or part from the mold cavity 18′.

[0049] As shown in cross-section 7-7 in FIG. 7, a plurality of groovesor slots 84 are provided in the base 81 of the pin member 80 in order toallow the gas to pass through the abutting surfaces.

[0050] Conduit 86 is also provided in the gas entry ejector pinmechanism 70 in order to allow gas from gas source 82 to return theejector pin to its initial or rest position. Gas flows through conduits86 and 87 and acts against the surface 81A of the base 81 of the pinmember 80 in order to return it to its initial position.

[0051] A plug member 73 is positioned in the mechanism 70 in order tohold the ejector pin member 80 in place.

[0052] In order to supply gas into the mold cavity 18′ during thegas-assisted injection molding process, gas from gas source 32′ issupplied through conduits 90 and 91. The pressurized gas will enter themold cavity 18′ through the clearance or space between the ejector pinmember 80 and the passageway or bore 94 in the member or mechanism 70 inwhich the ejector pin 80 is positioned. This is shown by arrows 96.

[0053] A plurality of seals 98 and 99 are provided in and around themember 70 in order to seal it from gas leaks. Seals 98 are preferablyO-ring seals with anti-rollover features.

[0054] The embodiment of gas entry ejector pin mechanism 70 eliminatesthe need to seal the entire ejector pin assembly or assemblies 34 andsaves gas. It also eliminates the ejector frame and saves material andcomponents. It also will reduce the mold height and assures a gas-tightmold.

[0055] In still another alternate embodiment, pressurized water can beused to form the cavity in the injected plastic material in place ofgas. For this purpose, a water pin assembly or assemblies is used in theplace of the gas pin assembly or assemblies 30 described above. Thewater under pressure can be inserted through the pin assembly into theplastic material as it is being injected into the mold cavity. The watercan come from a single charge, or the water, after forming the molding,can be circulated by exiting the pressurized water through a vent pin atthe end of the cavity.

[0056] The use of cool water will aid in removing heat from the plasticmaterial as it is being injected into the mold cavity and thus reducethe cycle time. Once the mold is self-supporting, the water flow can bestopped. Thereafter, gas can be introduced through the same pin assemblywhich will push water out of the molding. Also, a vacuum can be used todraw or remove the water from the molding system.

[0057] In a still further embodiment, sequencing of the gas pressurethrough the electrically controlled gas pressure valve 40 can beutilized. In this alternate process, when 95-98% by volume of theplastic material has been injected into the mold cavity, the gaspressure at gas valve 40 is held constant at its predetermined pressure.The excess gas is vented through the valve 40 as the cavity is beingfilled by the remaining plastic and gas is being introduced into themolding through the gas pin assembly or assemblies 30. The gas pressureat the pressure valve 40 will not exceed the predetermined pressureassembly when the cavity is filled.

[0058] The small amount of pressurized gas in the gas lines connectingthe valve 40 to the mold can be vented to atmosphere.

[0059] In a further alternate embodiment, the gas sequencing can beadjusted in a different manner. After the mold is closed and the gas ispressurized to pressure P1, the gas pressure in the mold cavity 18, willequal the pressure setting on electrically controlled gas pressure valve40. Plastic is then injected from the injection molding machine 20through the machine nozzle 22 into the mold cavity 18. Gas in the cavityis vented through the gas pressure valve 40 so that the gas pressure inthe cavity will never exceed the initial gas pressure. Thereafter, whenthe amount of plastic is 95-98% by volume of the prespecified amount,gas through gas pin assembly or assemblies 30 is initiated. Thereafter,gas pressure in the mold is held at the pressure P3 from the gas pinassembly 30. The remaining plastic is then introduced into the moldcavity to completely fill all of its extremities. Thereafter, when themolding is self-supporting, gas is vented through the gas pin assemblyand the part is removed from the mold.

[0060] While particular embodiments of the invention have been shown anddescribed, numerous variations and alternative embodiments will occur tothose skilled in the art. Accordingly, it is intended that the inventionbe limited only in terms of the appended claims.

What is claimed is:
 1. A method of forming an injection molded plasticpart in a mold, comprising: sealing the mold to prevent gas leakage fromthe mold cavity; applying an initial gas pressure in the mold cavity;injecting plastic material into the mold cavity; increasing the gaspressure in the mold cavity up to a preselected value; injecting gasinto the plastic material after 90%-99% of the preselected amount ofplastic material has entered the mold cavity; venting the gas from themold cavity at a controlled rate; allowing the plastic material toharden forming a hollow molded article; venting the gas from the hollowmolded article; and removing the completed plastic article from themold.
 2. The method of forming an injection molded plastic part asrecited in claim 1 wherein the initial gas pressure is supplied from afirst gas source and said injection of gas is supplied from a second gassource.
 3. The method of forming an injection molded plastic part asrecited in claim 2 wherein the first and second gas sources are thesame.
 4. The method of forming an injection molded plastic part asrecited in claim 1 wherein the step of removing the completed plasticpart from the mold comprises opening the mold and ejecting the part. 5.The method of forming an injection molded plastic part as recited inclaim 4 wherein the part is ejected from the mold by at least oneejector pin assembly.
 6. The method of forming an injection moldedplastic part as recited in claim 1 wherein the gas is injected into theplastic material by at least one gas pin assembly.
 7. The method offorming an injection molded plastic part as recited in claim 1 whereinthe gas pressure in the mold cavity is maintained at a preselected valueby a gas control mechanism.
 8. The method of forming an injection moldedplastic part as recited in claim 7 wherein said gas control mechanism isinfinitely adjustable.
 9. The method of forming an injection moldedplastic part as recited in claim 1 further comprising the step ofventing the injected gas from the plastic material before the part isremoved from the mold.
 10. The method of forming an injection moldedplastic part as recited in claim 9 wherein the injected gas is ventedthrough a gas pin assembly.
 11. The method of forming an injectionmolded plastic part as recited in claim 7 wherein said gas controlmechanism comprises a vent valve.
 12. A system for forming an injectionmolded plastic part in a mold comprising: a mold, said mold having apart-forming mold cavity therein; sealing members for sealing said moldcavity and preventing gas leakage therefrom; a first gas source forsupplying a gas into the mold cavity to pre-pressurize the mold cavityto a first pre-determined value; a vent valve for removing said gas fromthe mold cavity as desired; a gas control mechanism for maintaining thegas pressure in the mold cavity at a second predetermined value; asource for injecting molten plastic material into the mold cavity; a gaspin assembly for supplying gas into the plastic material in the moldcavity; and a second gas source for supplying gas to said gas pinassembly.
 13. The system as recited in claim 12 further comprising: atleast one ejector pin assembly for ejecting the completed plastic partfrom the mold cavity.
 14. The system as recited in claim 12 wherein saidfirst and second gas source are the same source.
 15. The system asrecited in claim 12 wherein said gas control mechanism comprises aninfinitely adjustable gas control valve.